Color image forming apparatus having drive current adjustment

ABSTRACT

A color image forming apparatus has a light emitting element emitting light, a laser driving unit causing the light emitting element to emit light of a light amount at a first emission level for visualizing a toner image onto a first area where the toner image is to be visualized on a charged photosensitive member and to emit light of a light amount at a second emission level for weak emission onto a second area where the toner is not to be adhered to the charged photosensitive member. In addition, an acquiring unit acquires information of an integrated number of rotations of the photosensitive member, a drive current adjusting unit adjusts the drive current for the second emission level, and changes a magnitude of the drive current for the second emission level in accordance with the information of the integrated number of rotations of the photosensitive member.

This application is a divisional of Application No. 13/472,935, filedMay 16, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a color image forming apparatusutilizing an electrophotographic recording system, such as a laserprinter, a copying machine, and a fax machine.

2. Description of the Related Art

Conventionally, there is known an image forming apparatus such as acopying machine and a laser printer, which use an electrophotographicrecording system. In the image forming apparatus utilizing theelectrophotographic recording system, for example, the followingelectrophotographic process is executed. First, a surface of aphotosensitive drum is uniformly charged to, for example, −600 V by acharging device. After that, laser emission is performed by a laserexposure device to form an electrostatic latent image on thephotosensitive drum. Then, toner image is adhered to the electrostaticlatent image by a developing device to form a toner image, and the tonerimage is transferred onto a transfer member by a transfer device.

Residual toner on the photosensitive drum is removed by a drum cleaningdevice. Japanese Patent Application Laid-Open No. 2001-281944 describesthe idea of removing the residual potential on the photosensitive drumby the light irradiation with a pre-exposure lamp to serve thephotosensitive drum for the subsequent image formation.

SUMMARY OF THE INVENTION

In the electrophotographic image forming apparatus, in forming theelectrostatic latent image on the surface of the photosensitive drum,prior control of a charge potential of the surface of the photosensitivedrum is important. Particularly in the case of a color printer, it isnecessary to cope with variability in photosensitivity characteristicsof the respective photosensitive drums caused by the individualdifference and usage difference thereof. As for the control of thecharge potential, various control methods are proposed, such as a methodusing the pre-exposure lamp described above, but from the viewpoint ofcost reduction and downsizing of the apparatus main body, a simplerconfiguration is desired.

The present invention has an object to solve at least one of theabove-mentioned problem and other problems. For example, the presentinvention has an object to solve problems caused by a charge potentialof a photosensitive drum by coping with variability or variations inphotosensitivity characteristics (EV curve characteristics) of thephotosensitive drums in an apparatus, and appropriately controlling thecharge potential of each of the photosensitive drums with a simplerconfiguration.

According to an exemplary embodiment of the present invention, a colorimage forming apparatus which has a plurality of photosensitive membersto be charged and a light irradiation unit irradiating at least one ofthe charged photosensitive members with light to form electrostaticlatent image, the color image forming apparatus comprising: a laserdriving unit causing the light irradiation unit to emit light inaccordance with an input of print data, the laser driving unit causingthe light irradiation unit to emit light of a light amount at a firstemission level for printing when exposing an image portion and to emitlight of a light amount at a second emission level for weak emissionwhen exposing a non-image portion; an acquiring unit acquiringinformation associated with remaining lifetime of each of thephotosensitive members; a first light intensity adjusting unit adjustinga first drive current for causing the light irradiation unit to emitlight at the first emission level; and a second light intensityadjusting unit adjusting a second drive current for causing the lightirradiation unit to emit light at the second emission level, the secondlight intensity adjusting unit changing a magnitude of the second drivecurrent in accordance with the information associated with the remaininglifetime of each of the photosensitive members acquired by the acquiringunit.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically illustrates a cross-section of a color imageforming apparatus.

FIG. 1B illustrates a cross-section of a photosensitive drum.

FIG. 2 illustrates an example of sensitivity characteristics (EV curve)of the photosensitive drum.

FIGS. 3A and 3B each illustrate a high voltage power supply circuit forcharging rollers and developing rollers.

FIG. 4 illustrates an example of an appearance of an optical scanningdevice.

FIG. 5 illustrates an example of a laser drive circuit having atwo-level light intensity adjusting function.

FIGS. 6A and 6B illustrate a relationship between a current flowingthrough a laser diode and emission intensity.

FIG. 7 illustrates another example of the laser drive circuit having thetwo-level light intensity adjusting function.

FIG. 8 is a timing chart of automatic light amount control.

FIGS. 9A, 9B and 9C each illustrate a relationship between weak emissionand PWM emission.

FIGS. 10A, 10B and 10C illustrate a relationship among a photosensitivedrum film thickness, a charge potential, a development potential, and anexposure potential.

FIG. 11 is a flowchart illustrating setting processing of a generalexposure parameter and a weak exposure parameter, image formationprocessing, and update processing of photosensitive drum usage.

FIG. 12 shows an example of a table in which the photosensitive drumusage is associated with the general exposure parameter and the weakexposure parameter.

FIG. 13 illustrates an effect related to a fog amount.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the present invention aredescribed in detail with reference to the drawings. Note that,components described in the embodiments are merely an example, and thescope of the present invention is not limited to only those embodiments.

(First Embodiment)

Referring to FIGS. 1A to 10C, a configuration of a color image formingapparatus (hereinafter, simply referred to as an image formingapparatus) is described, and then, referring to FIGS. 11 and 12, controloperation of weak exposure is described. Finally, referring to FIG. 13,an effect related to a fog amount is described.

(Schematic View of Cross-Section of Image Forming Apparatus)

FIG. 1A schematically illustrates a cross-section of the image formingapparatus. Referring to FIG. 1A, a configuration and operation of theimage forming apparatus according to a first embodiment are described.The image forming apparatus includes first to fourth image formingstations a to d. The first image forming station is for yellow(hereinafter, represented by “Y”), the second image forming station isfor magenta (hereinafter, represented by “M”), the third image formingstation is for cyan (hereinafter, represented by “C”), and the fourthimage forming station is for black (hereinafter, represented by “Bk”).Each of the image forming stations a to d includes a storage member(memory tag) (not shown) for storing a cumulative number of rotations ofeach of photosensitive drums 1 a to 1 d as information on lifetime ofthe photosensitive drum. Note that, other than the case wheredescription for each color is necessary, reference symbols a to d areomitted as appropriate. Each of the image forming stations isreplaceable with respect to a main body of the image forming apparatus.Further, each of the image forming stations is only required to includeat least the photosensitive drum 1. The range of the members to beincluded in the image forming station so as to be replaceable is notparticularly limited.

In the following, an operation of a first image forming station (Y) a isdescribed as a representative example of the image forming stations. Theimage forming station a includes the photosensitive drum 1 a serving asa photosensitive member. The photosensitive drum 1 a is driven so as torotate at a predetermined circumferential speed (process speed) in adirection of the arrow. In this rotation process, the photosensitivedrum 1 a is uniformly charged by a charging roller 2 a to a chargepotential having a predetermined polarity. Subsequently, throughscanning of a laser beam 6 a from an exposure device 31 a serving as anexposure unit based on image data (image signal) supplied from outside,a surface of the photosensitive drum 1 a corresponding to an imageportion is exposed with light to remove charges, and thus an exposurepotential Vl (VL) is formed on the surface of the photosensitive drum 1a. Subsequently, on an exposure potential Vl (VL) portion correspondingto the image portion, toner is developed due to a potential differencebetween the exposure potential Vl (VL) and a development potential Vdcapplied to a developing device (yellow developing device) 4 a serving asa first developing unit, to thereby be visualized. The image formingapparatus of this embodiment is a reversal development type imageforming apparatus which performs image exposure by the exposure device31 a to develop the toner on the exposed portion.

An intermediate transfer belt 10 is suspended by suspension members 11,12, and 13, and abuts against the photosensitive drum 1 a. Theintermediate transfer belt 10 is driven to rotate in the same directionand at substantially the same circumferential speed as thephotosensitive drum 1 a at an abutment position. A yellow toner imageformed on the photosensitive drum 1 a is transferred as follows. In aprocess in which the yellow toner image passes through an abutmentportion (hereinafter, referred to as primary transfer nip portion)between the photosensitive drum 1 a and the intermediate transfer belt10, with a primary transfer voltage applied to a primary transfer roller14 a by a primary transfer power supply 15 a, the yellow toner image istransferred onto the intermediate transfer belt 10 (primary transfer).Primary transfer residual toner remaining on the surface of thephotosensitive drum 1 a is cleaned and removed by a drum cleaner 5 aserving as a cleaning unit. After that, the image forming process fromthe charging and the steps following the charging described above isrepeated. Similarly, a magenta toner image (M) with the second color, acyan toner image (C) with the third color, and a black toner image (Bk)with the fourth color are formed, and the toner images are transferredonto the intermediate transfer belt 10 while being sequentiallyoverlapped one on top of another, thereby obtaining a composite colorimage.

In a process in which the four-color toner images on the intermediatetransfer belt 10 pass through an abutment portion (hereinafter, referredto as secondary transfer nip portion) between the intermediate transferbelt 10 and a secondary transfer roller 20, a secondary transfer powersupply 21 applies a secondary transfer voltage to the secondary transferroller 20. With this, the four-color toner images on the intermediatetransfer belt 10 are collectively transferred onto a surface of arecording material P fed from a sheet feeding roller 50. After that, therecording material P bearing the four-color toner images is introducedinto a fixing device 30 to be heated and pressurized, and thus thefour-color toners are melted and mixed to be fixed onto the recordingmaterial P. With the above-mentioned operation, a full-color toner imageis formed on a recording medium. Secondary transfer residual tonerremaining on the surface of the intermediate transfer belt 10 is cleanedand removed by an intermediate transfer belt cleaning device 16.

Description is made with reference to FIG. 1A of an example of the imageforming apparatus including the intermediate transfer belt 10, but thepresent invention is not limited thereto. For example, the presentinvention may be implemented by utilizing an image forming apparatus,which includes a recording material conveyance belt (on a recordingmaterial carrier) and employs a method in which the toner imagedeveloped on the photosensitive drum is directly transferred onto therecording material conveyed by the recording material conveyance belt.Hereinafter, the image forming apparatus including the intermediatetransfer belt 10 is described as an example.

(Cross-Section of Photosensitive Drum)

FIG. 1B illustrates an example of a cross-section of the photosensitivedrum 1 a. In the photosensitive drum 1 a, a charge generating layer 23 aand a charge transporting layer 24 a are laminated on a conductivesupport base member 22 a. The conductive support base member 22 a is analuminum cylinder having an outer diameter of 30 mm and a thickness of 1mm, for example. The charge generating layer 23 a is a layer of aphthalocyanine-based pigment having a thickness of 0.2 μm, for example.The charge transporting layer 24 a is a layer having a thickness of 20μm, in which polycarbonate is used as a binder resin and an aminecompound is contained as a charge transporting material, for example.FIG. 1B is an example of the photosensitive drum 1 a, and dimensions andmaterials therefor are not limited to those described herein.

(Sensitivity Characteristics of Photosensitive Drum)

FIG. 2 is a graph illustrating an example of an EV curve representingthe photosensitivity characteristics of the photosensitive drum 1, inwhich the lateral axis represents an exposure amount E (μJ/cm²) and thevertical axis represents the potential of the photosensitive drum 1(photosensitive drum potential) (V). Vcdc represents a charge voltage,and this graph illustrates a case where −1,100 V is applied as thecharge voltage Vcdc. FIG. 2 illustrates a potential decay in a casewhere the photosensitive drum 1 after charging, which has a surfacecharged to V, is exposed with laser beams so that the exposure amount isE (μJ/cm²) on the surface of the photosensitive drum. The EV curve showsthat a larger potential decay is obtained by increasing the exposureamount E. A high potential portion has an intense electric fieldenvironment, and hence recombination of charge carriers (electron-holepair) generated by the exposure is less likely to occur. Therefore, alarge potential decay is exhibited even with a small exposure amount. Onthe other hand, in a low potential portion, generated carriers areeasily recombined, and hence there is observed a phenomenon that apotential decay is small even with an exposure of a large exposureamount. FIG. 2 illustrates an EV curve at an initial stage of usage ofthe photosensitive drum, and an EV curve when the photosensitive drum iscontinuously used. In FIG. 2, the curve in the broken line is the EVcurve of the photosensitive drum with the number of rotations r of75,000≦r<112,500, for example. The sensitivity characteristics of thephotosensitive drum illustrated in FIG. 2 are an example, andphotosensitive drums with various EV curves may be applied in the firstembodiment.

(Regarding Charge/Development High Voltage Power Supply 52)

Next, referring to FIGS. 3A and 3B, a charge/development high voltagepower supply is described. FIGS. 3A and 3B illustrate examples of thecharge/development high voltage power supply. In the example of FIG. 3A,charging rollers 2 a to 2 d respectively corresponding to the multiplecolors, and developing rollers 43 a to 43 d respectively correspondingto the multiple colors are connected to a charge/development highvoltage power supply 52. The charge/development high voltage powersupply 52 supplies the charge voltage Vcdc (power supply voltage) outputfrom one transformer 53 to the charging rollers 2 a to 2 d. Further, thecharge/development high voltage power supply 52 supplies a developmentvoltage Vdc obtained through division by two resistor elements R3 and R4to the developing rollers 43 a to 43 d. In the power supply circuitsillustrated in FIGS. 3A and 3B, the power supply system is simplified,and hence voltages input (applied) to the respective rollers can becollectively adjusted while maintaining a predetermined relationship.Meanwhile, individual adjustment (individual control) cannot beperformed independently among the colors. The same applies to thedeveloping roller 43.

Here, the resistor elements R3 and R4 may be configured by any one of afixed resistor, a pre-set variable resistor, and a variable resistor. InFIG. 3A, the power supply voltage itself from the transformer 53 isdirectly input to the charging rollers 2 a to 2 d, and a divided voltageobtained by dividing the voltage output from the transformer 53 by thefixed voltage dividing resistors is directly input to the developingrollers 43 a to 43 d. However, this is merely an example, and thevoltage input form is not limited thereto. Various forms for inputting avoltage to the individual rollers (charging unit and developing unit)may be employed.

For example, instead of the output itself from the transformer 53, thefollowing voltage may be output. That is, a conversion voltage(post-conversion voltage) obtained by DC-DC converting the output fromthe transformer 53 by a converter, or a voltage obtained by at least oneof dividing and dropping the power supply voltage or the conversionvoltage by an electronic element having fixed voltage dropcharacteristics may be input to the charging rollers 2 a to 2 d.Further, the conversion voltage obtained by DC-DC converting the outputfrom the transformer 53 by the converter, or the voltage obtained by atleast one of dividing and dropping the power supply voltage or theconversion voltage by the electronic element having the fixed voltagedrop characteristics may be input to the developing rollers 43 a to 43d. Examples of the electronic element having the fixed voltage dropcharacteristics include a resistor element and a zener diode. Theconverter includes an adjustable regulator. At least one of dividing anddropping a voltage by the electronic element includes a case of furtherdropping the divided voltage, and an inverse case thereof, for example.

Meanwhile, in order to control the charge voltage Vcdc to besubstantially constant, a negative voltage obtained by dropping thecharge voltage Vcdc by R2/(R1+R2) is offset by a reference voltage Vrgvtoward a positive polarity to be set as a monitor voltage Vref, andfeedback control is performed so that the monitor voltage Vref is keptat a constant value. Specifically, a control voltage Vc preset in anengine controller 122 (CPU) (see FIG. 5) is input to a positive terminalof an operational amplifier 54, whereas the monitor voltage Vref isinput to a negative terminal thereof. The engine controller 122 changesthe control voltage Vc as appropriate depending on situations at eachtime. Then, the control/drive system of the transformer 53 isfeedback-controlled by an output value of the operational amplifier 54so that the monitor voltage Vref becomes equal to the control voltageVc. With this, the charge voltage Vcdc output from the transformer 53 iscontrolled to become a target value. Regarding the output control of thetransformer 53, the output of the operational amplifier 54 may be inputto the CPU, and the results of the computation by the CPU may bereflected to the control/drive system of the transformer 53. In thisembodiment, control is performed so that the charge voltage Vcdc is−1,100 V, and the development voltage Vdc is −350 V. Under this control,the charging rollers 2 a to 2 d uniformly charge the surfaces of thephotosensitive drums 1 a to 1 d, respectively, to a charge potential Vd.

Further, FIG. 3B illustrates another example of the charge/developmenthigh voltage power supply. The same components as those of FIG. 3A aredenoted by the same reference symbols, and description thereof isomitted. In FIG. 3B, the power supply is divided into at least two powersupplies, that is, a charge/development high voltage power supply 90 forthe image forming stations of YMC colors and a charge/development highvoltage power supply 91 for the image forming station of a Bk color.When an image is formed in a full-color mode, the charge/developmenthigh voltage power supplies 90 and 91 are turned ON. When an image isformed in a monochrome mode, the charge/development high voltage powersupply 90 for the image forming stations of YMC colors is not operated(turned OFF), and the charge/development high voltage power supply 91for the image forming station of a Bk color is turned ON. In the case ofFIG. 3B, the same description as in the case of FIG. 3A applies to thecharge/development high voltage power supply 90 for the image formingstations of YMC colors.

With use of the charge/development high voltage power supplies of FIGS.3A and 3B, because the high voltage power supply is shared among themultiple charging rollers and developing rollers, further downsizing ofthe apparatus can be realized. Further, the cost can be suppressed ascompared to the case where transformers which output variable voltagesare provided for the respective colors, and the voltages to be input tothe respective charging rollers and the respective developing rollersare individually controlled. In addition, the cost can be suppressed ascompared to the case where DC-DC converters (adjustable regulators) areprovided to the respective charging rollers and the respectivedeveloping rollers, thereby controlling the output from one transformerindividually for the respective charging rollers and the respectivedeveloping rollers.

(Appearance of Light Scanning Device)

FIG. 4 is a view of a representative appearance of a light scanningdevice. Through a laser diode 107 (hereinafter, referred to as LD 107)serving as a light emitting element, a drive current flows owing to theoperation of a laser drive system circuit 130. The LD 107 emits laserbeams at an intensity level corresponding to the drive current. Thelaser drive system circuit 130 (hereinafter, referred to as LD driver130) is a circuit for driving the LD 107 electrically connected to theengine controller 122 and a video controller 123 to be described later.The laser beams emitted by the LD 107 are shaped by a collimator lens134, and are aligned as parallel beams to be scanned in a horizontaldirection of the photosensitive drum 1 by a polygon mirror 133. Thescanned laser beams are imaged by an fθ lens 132 on the surface of thephotosensitive drum 1 rotating in a direction of the arrow about arotation shaft thereof. Thus, dot exposure is performed.

A reflective mirror 131 is provided so as to correspond to a scanningposition of the photosensitive drum 1 on one end side thereof, tothereby reflect the laser beam projected to a scanning start positiontoward a BD synchronization detection sensor 121 (hereinafter, referredto as BD detection sensor). With the output of the BD detection sensor121, the scanning start timing of the laser beam is determined. At thetime of forced emission in detection of the laser beam, auto powercontrol (APC) is performed, which is automatic light amount control forcontrolling the laser light amount to a desired light amount, and thusthe emission level of the laser is adjusted.

(Laser Drive System Circuit)

FIG. 5 is a laser drive system circuit for automatically adjusting thelight amount level of the LD 107 for the purpose of performing weakemission so as to prevent adhesion of toner on the photosensitive drum 1and prevent the positive or reversal fog in the non-image portion. InFIG. 5, the LD driver 130 illustrated in FIG. corresponds to a portionsurrounded by a dotted line frame 130 a. Configurations in dotted lineframes 130 b to 130 d of FIG. 5 are the same as that in the dotted lineframe 130 a. The configurations in the dotted line frames 130 a to 130 dcorrespond to LD drivers for respective colors in the color imageforming apparatus. Hereinafter, the configuration of the LD driver 130for a specific color is described. The LD drivers 130 for the othercolors have the same configuration, and hence overlapping description isomitted. Hereinafter, description is made of FIG. 5.

The LD driver 130 includes PWM smoothing circuits 140 and 150 (alternatelong and short dash lines), comparator circuits 101 and 111, sample/holdcircuits 102 and 112, and hold capacitors 103 and 113. The LD driver 130further includes current amplifier circuits 104 and 114, referencecurrent sources (constant current circuits) 105 and 115, switchingcircuits 106 and 116, and a current voltage conversion circuit 109.Hereinafter, a photodiode 108 is referred to as PD 108. The comparatorcircuit 101, the sample/hold circuit 102, the hold capacitor 103, thecurrent amplifier circuit 104, the reference current source 105, and theswitching circuit 106 correspond to a first light intensity adjustingportion. The comparator circuit 111, the sample/hold circuit 112, thehold capacitor 113, the current amplifier circuit 114, the referencecurrent source 115, and the switching circuit 116 correspond to a secondlight intensity adjusting portion. An emission level for normal printingand an emission level for weak emission, which are described later, areindependently controllable by the first light intensity adjustingportion and the second light intensity adjusting portion, respectively.

The engine controller 122 incorporates an ASIC, a CPU, a RAM, and anEEPROM. Further, the engine controller 122 performs printer enginecontrol and control of communication to the video controller 123.

The engine controller 122 outputs a PWM signal PWM1 to the PWM smoothingcircuit 140. The PWM smoothing circuit 140 includes an inverter circuit141, resistors 142 and 144, and a capacitor 143. The inverter circuit141 inverts the PWM signal PWM1. An output of the inverter circuit 141charges the capacitor 143 via the resistor 142, and is smoothed by thecapacitor 143 to be a voltage signal. The smoothed voltage signal isinput to a terminal of the comparator circuit 101 as a reference voltageVref11. The reference voltage Vref11 is determined by a pulse width ofthe PWM signal PWM1, and is controlled by the engine controller 122.

The engine controller 122 outputs a PWM signal PWM2 to the PWM smoothingcircuit 150. The PWM smoothing circuit 150 includes an inverter circuit151, resistors 152 and 154, and a capacitor 153. The inverter circuit151 inverts the PWM signal PWM2. An output of the inverter circuit 151charges the capacitor 153 via the resistor 152, and is smoothed by thecapacitor 153 to be a voltage signal. The smoothed voltage signal isinput to a terminal of the comparator circuit 111 as a reference voltageVref21. The reference voltage Vref21 is determined by a pulse width ofthe PWM signal PWM2, and is controlled by the engine controller 122.Both the reference voltages Vref11 and Vref21 may be directly outputwithout an instruction of the PWM signal from the engine controller 122.

An OR circuit 124 has input terminals to which an Ldry signal from theengine controller 122 and a VIDEO signal from the video controller 123are input. The OR circuit 124 outputs a Data signal to the switchingcircuit 106. The VIDEO signal is based on print data sent from anexternal device, such as a reader scanner which is externally connectedor a host computer. The VIDEO signal is driven by, for example, imagedata having an 8-bit (=256 levels) multivalued signal (0 to 255), anddetermines a laser emission period. A pulse width when the image data is0 (background portion) is PW_(MIN) (for example, 0.0% of a pulse widthfor one pixel). When the image data is 255, which means full exposure,the pulse width corresponds to one pixel (PW₂₅₅). Regarding image dataof values ranging from 1 to 254, a pulse width (PW_(X)) proportional toa gradation value between PW_(MIN) and PW₂₅₅ is generated, which isexpressed by Expression (1).PW_(n) =n×(PW₂₅₅−PW_(MIN))/255+PW_(MIN)  Expression (1)

The case where the image data for controlling the laser diode 107 is 8bits (=256 levels) is an example. For example, the image data may be amultivalued signal of 4 bits (=16 levels) or 2 bits (=4 levels) afterhalftone processing. The image data after halftone processing may be abinary signal.

The VIDEO signal, which is output from the video controller 123, isinput to a buffer 125 with enable terminal (ENB). An output of thebuffer 125 is input to the OR circuit 124. The enable terminal isconnected to a signal line to which a Venb signal from the enginecontroller 122 is output.

The engine controller 122 outputs an SH1 signal, an SH2 signal, a Basesignal, the Ldry signal, and the Venb signal. The Venb signal is asignal for subjecting the Data signal based on the VIDEO signal to maskprocessing. Through setting of the Venb signal to a disabled state (OFFstate), a timing for an image mask region (image mask period) can beproduced.

To positive terminals of the comparator circuits 101 and 111, the firstreference voltage Vref11 and the second reference voltage Vref21 areinput, respectively. Outputs of the comparator circuits 101 and 111 areinput to the sample/hold circuits 102 and 112, respectively. Thereference voltage Vref11 is set as a target voltage for causing the LD107 to emit light at the emission level for normal printing (firstemission level or first light amount). The reference voltage Vref21 isset as a target voltage for the emission level for weak emission (secondemission level or second light amount). The sample/hold circuits 102 and112 are connected to the hold capacitors 103 and 113, respectively.Outputs of the sample/hold circuits 102 and 112 are input to positiveterminals of the current amplifier circuits 104 and 114, respectively.

The current amplifier circuits 104 and 114 are connected to thereference current sources 105 and 115, respectively. Outputs of thecurrent amplifier circuits 104 and 114 are input to the switchingcircuits 106 and 116, respectively. To negative terminals of the currentamplifier circuits 104 and 114, a third reference voltage Vref12 and afourth reference voltage Vref22 are input, respectively. A current Io2(first drive current) is determined in accordance with a differencebetween an output voltage of the sample/hold circuit 102 and thereference voltage Vref12. A current Io2 (second drive current) isdetermined in accordance with a difference between an output voltage ofthe sample/hold circuit 112 and the reference voltage Vref22. Thereference voltages Vref12 and Vref22 are set for determining thecurrents.

The switching circuit 106 is turned ON and OFF based on the Data signal,which is a pulse modulated data signal. The switching circuit 116 isturned ON and OFF based on the input signal Base. Output terminals ofthe switching circuits 106 and 116 are connected to a cathode of the LD107, thereby supplying drive currents Idrv and Ib. An anode of the LD107 is connected to a power supply Vcc. A cathode of the photodiode 108(hereinafter, referred to as PD 108) for monitoring the light amount ofthe LD 107 is connected to the power supply Vcc. An anode of the PD 108is connected to the current voltage conversion circuit 109, therebycausing a monitor current Im to flow through the current voltageconversion circuit 109. With this, the current voltage conversioncircuit 109 converts the monitor current Im into a monitor voltage Vm.The monitor voltage Vm is negatively fed back to negative terminals ofthe comparator circuits 101 and 111.

In FIG. 5, the engine controller 122 and the video controller 123 areillustrated separately, but the present invention is not limited to thisform. For example, a part or whole of the engine controller 122 and thevideo controller 123 may be constituted by the same controller.Similarly, a part or whole of the LD driver 130 surrounded by the dottedline frame of FIG. 5 may be built in the engine controller 122, forexample.

(Description of APC for P(Idrv))

Next, the APC for P(Idrv) is described. The engine controller 122 setsthe sample/hold circuit 112 to a hold state (non-sampling period) by aninstruction of the SH2 signal, and sets the switching circuit 116 to anoperation OFF state by the input signal Base. The engine controller 122sets the sample/hold circuit 102 to a sampling state by an instructionof the SH1 signal, and turns ON the switching circuit 106 by the Datasignal. More specifically, at this time, the engine controller 122controls (instructs) the Ldry signal to set the Data signal so as toobtain an emission state of the LD 107. This period during which thesample/hold circuit 102 is in the sampling state corresponds to an APCoperation period.

Under this state, when the LD 107 becomes a full emission state, the PD108 monitors the emission intensity (emission amount) of the LD 107, anda monitor current Im1 proportional to the emission intensity flows.Then, the monitor current Im1 is caused to flow through the currentvoltage conversion circuit 109, and the current voltage conversioncircuit 109 converts the monitor current Im1 into a monitor voltage Vm1.The current amplifier circuit 104 controls the drive current Idrv basedon the current Io1 flowing through the reference current source 105 sothat the monitor voltage Vm1 matches the first reference voltage Vref11,which is a target value.

During a non-APC operation period, that is, during normal imageformation, the sample/hold circuit 102 is in a hold period (non-samplingperiod). The switching circuit 106 is turned ON and OFF in accordancewith the Data signal, thereby subjecting the drive current Idrv to pulsewidth modulation.

(Description of APC for P(Ib))

Next, the APC for P(Ib) is described. The engine controller 122 sets thesample/hold circuit 102 to a hold state (non-sampling period) by aninstruction of the SH1 signal, and sets the switching circuit 106 to anoperation OFF state by the Data signal. Regarding the Data signal, theengine controller 122 sets the Venb signal, which is connected to theenable terminal of the buffer with enable terminal 125, to a disabledstate, and controls the Ldry signal to turn OFF the Data signal.Further, the engine controller 122 sets the sample/hold circuit 112 to asampling state (that is, an APC operation period) by an instruction ofthe SH2 signal, and turns ON the switching circuit 116 by the inputsignal Base so that the LD 107 is set to a weak emission state.

Under this state, when the LD 107 becomes a full weak emission state(lighting maintained state) in which the light amount is weak, the PD108 monitors the emission intensity of the LD 107, and a monitor currentIm2 (Im1>Im2) proportional to the emission intensity flows. Then, themonitor current Im2 is caused to flow through the current voltageconversion circuit 109, and the current voltage conversion circuit 109converts the monitor current Im1 into a monitor voltage Vm2. The currentamplifier circuit 114 controls the drive current Ib based on the currentIo2 flowing through the reference current source 115 so that the monitorvoltage Vm2 matches the second reference voltage Vref21, which is atarget value.

During the non-APC operation period, that is, during the normal imageformation (period in which an image signal is sent), the sample/holdcircuit 112 is in a holding period (non-sampling period), and the fullweak emission state in which the light amount is weak is maintained.

If the positive fog, the reversal fog, and the like of toner can beneglected, it is only necessary to set the laser emission amount forweak emission to an appropriate laser emission amount (intensity) atwhich the charge potential does not fall below the developmentpotential, but this is impractical. That is, when the positive fog, thereversal fog, and the like of toner are taken into consideration, it isnecessary to keep the light amount for P(Ib) always stable during imageformation.

(Description of Weak Emission Level)

In the description above, the drive current Ib under the full weakemission state is set so as to exceed a threshold current Ith of the LD107 illustrated in FIG. 6A to be the weak emission level P(Ib). FIG. 6Ais a graph illustrating a relationship between each laser emissionintensity and each current value. The weak emission level means anemission intensity level at which a developing material such as toner ispractically not adhered by charging (not visualized) on thephotosensitive drum by laser irradiation at that level, and at which atoner fog state is suppressed. The emission intensity of the emissionlevel P(Ib) is in a laser emission region. If the emission level P(Ib)at this time is in an LED emission region in which the emission level isbelow the emission level of the laser emission region, a wavelengthdistribution in a spectrum significantly spreads, resulting in awavelength distribution which is wider than the rated laser wavelength.Accordingly, the sensitivity of the photosensitive drum is disturbed,and the surface potential becomes unstable. Therefore, the emissionlevel P(Ib) needs to be in the laser emission region whose emissionlevel exceeds that of the LED emission region.

During the normal image formation, the drive current Idrv+Ib is set soas to be the emission level corresponding to the intensity of a printlevel P(Idrv+Ib). The print level means an emission intensity level atwhich the charge adhesion of a developing material on the photosensitivedrum becomes saturated.

Vcdc (charge voltage), which has been described with reference to FIGS.3A and 3B, is set to be variable depending on parameters such as theenvironment and the degree of deterioration (usage) of thephotosensitive drum. From the viewpoint of maintaining image quality, itis also necessary to set the light amount of the target weak emissionlevel (intensity of the second emission level) to be variable dependingon the same parameters. For example, as the value of Vcdc becomeslarger, a light amount of a weak emission level Ebg1 also becomeslarger, and, on the other hand, as the value of Vcdc becomes smaller,the light amount of the weak emission level Ebg1 also becomes smaller.Details are described later.

(Description of Light Emission at P(Ib+Idrv))

In a case where the LD 107 is caused to emit light at the emission levelfor normal printing, the circuit of FIG. 5 is operated as follows. Thesample/hold circuit 112 is set to a hold period, and the switchingcircuit 116 is turned ON. Further, the sample/hold circuit 102 is set toa hold period, and the switching circuit 106 is turned ON. With this,the drive current Idrv+Ib is supplied. When the switching circuit 106 isturned OFF, the weak emission level P(Ib) of the drive current Ib isobtained.

The print level P(Idrv+Ib) is an emission intensity obtained bysuperimposing, on the weak light emission level P(Ib), the PWM lightemission level P(Idrv) resulting from pulse width modulation. Morespecifically, the switching circuit 106 is turned ON and OFF by the Datasignal (VIDEO signal) under a state in which SH2 and SH1 are set to thehold period and the Base signal is set to be ON and in which the enginecontroller 122 sets the Venb signal to an enabled state. With this,two-level emission can be performed between Ib and Idrv+Ib in terms ofdrive current, that is, between P(Ib) and P(Idrv+Ib) in terms ofemission intensity (see arrows of FIG. 6A). Further, regarding the lightamount at P(Idrv+Ib), laser emission is performed based on P(Ib) for atime period in accordance with a pulse duty.

Through the operation of the circuit of FIG. 5, the engine controller122 can perform the APC of the LD 107 for the weak emission level,thereby causing the LD 107 to emit light at the weak emission levelP(Ib). Further, with the use of the Data signal resulting from the VIDEOsignal sent from the video controller 123, light can be emitted at theprint level P(Idrv+Ib), which is the first level in the laser emissionregion. In this manner, the two emission levels can be obtained.

(Another Laser Drive System Circuit)

A circuit of FIG. 7 is different from the circuit of FIG. 5 in that aresistor Rb for causing a bias current Ibias to flow is added. The biascurrent Ibias is set to be lower than the threshold current Ith of theLD 107, and is set within a non-laser emission region (generally callednormal LED emission region). FIG. 6B illustrates a relationship betweeneach laser emission intensity and each current value. The effect of thebias current includes, as introduced in various documents, theimprovement of warm-up characteristics of the LD 107.

In the circuit of FIG. 7, the sample/hold circuit 112 is set to a holdstate by the SH2 signal, and the switching circuit 116 is turned ON, tothereby supply a drive current (Ib+Ibias) to the LD 107. In the circuitof FIG. 7, at this time, the LD 107 emits light at a weak emission levelP(Ib+Ibias). In this case, the emission level P(Ib+Ibias) is in thelaser emission region. Further, the sample/hold circuit 102 is set to ahold period by the SH1 signal, and the switching circuit 106 is turnedON by the Data signal, to thereby further supply the drive current Idrv.With this, a drive current (Idrv+Ib+Ibias) in total is supplied, andlight is emitted at an emission level P(Idrv+Ib+Ibias) for normalprinting.

As described above, the LD 107 is switched by the ON/OFF operation ofthe switching circuit 106 between emitting light at the emissionintensity of the print level P(Idrv+Ib+Ibias) and emitting light at theweak emission level P(Ib+Ibias) with the drive current (Ib+Ibias). Morespecifically, the switching circuit 106 is turned ON and OFF by the Datasignal resulting from the VIDEO signal under a state in which SH2 andSH1 are set to the hold period and the Base signal is set to be ON andin which the engine controller 122 sets the Venb signal to an enabledstate. With this, PWM laser emission can be performed with two-levelemission states between (Ib+Ibias) and (Idrv+Ib+Ibias) in terms of drivecurrent, that is, between P(Ib+Ibias) and P(Idrv+Ib+Ibias) in terms ofemission intensity (see arrows of FIG. 6B).

(Two-Level APC Sequence)

Next, execution timings of various kinds of processing related to theAPC for maintaining the laser emission level are described. FIG. 8 is anexample of a timing chart related to laser scanning. First, at a timingts, the engine controller 122 turns ON the SH1 signal and the Ldrysignal, and turns ON the switching circuit 106. Hereinafter, the phrase“timing ts” is simply referred to as “ts”, and the same applies to othersuch phrases. Then, an output of the BD detection sensor 121 is outputas a horizontal synchronization signal /BD at tb0. At tb0, thehorizontal synchronization signal /BD is detected by the enginecontroller 122, and then, at tb1, the engine controller 122 turns OFFboth the SH1 signal and the Ldry signal, and turns OFF the switchingcircuit 106. This finishes the APC for the normal print level. Then,after the APC for the print level is finished, laser emission at thenormal print level is performed by the LD 107 in accordance with theVIDEO signal. Then, the laser emission in accordance with the VIDEOsignal is performed in a period between tb1 and tb2, but detaileddescription thereof is omitted.

The engine controller 122 adjusts Io1 (first drive current) withreference to the output timing (detection timing) of the horizontalsynchronization signal /BD corresponding to the previous scanning line.More specifically, the engine controller 122 turns ON the SH1 signal andthe Ldry signal and turns ON the switching circuit 106 at tb2 after apredetermined period of time has elapsed since the output timing (tb0 ortb1) of the horizontal synchronization signal /BD (before detection ofthe next horizontal synchronization signal /BD). In response thereto,the APC for the print level is started again. Upon the start of the APC,the engine controller 122 turns OFF the Venb signal, and inputs adisable instruction to the enable terminal of the buffer 125. Thedisable instruction has similarly been input in the previous APC. Withthis, even if the video controller 123 has an erroneous output(containing noise or the like), an APC-related control instruction fromthe engine controller 122 can be reflected on the control.

The output of the BD detection sensor 121 is output as the horizontalsynchronization signal /BD at t0. At t0, the horizontal synchronizationsignal /BD is detected by the engine controller 122, and then, at t1,the SH1 signal and the Ldrv signal is turned OFF, and the switchingcircuit 106 is turned OFF, and thereby the APC for the normal printlevel is finished again.

Subsequently, at t1 after the detection of the horizontalsynchronization signal /BD, the engine controller 122 turns ON the SH2signal and the Base signal, and turns ON the switching circuit 116. Inresponse thereto, the engine controller 122 starts the APC for the weakemission level. The start timing of the APC for the weak emission levelmay be after t1 and before t2. It is only necessary to perform the APCfor the weak emission level in at least a part of the image mask period,which is after t1 and before t2. In particular, it is effective toexecute the APC for the weak emission level in a margin portion periodbetween t2 to t3. The engine controller 122 turns ON the SH2 signaluntil t3. In other words, the engine controller 122 maintains the APCfor the weak emission level until t3. With this, a longer time period ofthe APC for the weak emission level can be secured. A paper end portiontiming is t2, and there is a relationship of t1<t2<t3.

FIG. 9A illustrates a transition of the emission intensity of the LD107. Further, FIG. 9B illustrates a transition of the emission intensityof the LD 107 during the PWM type weak emission. In the PWM type weakemission of FIG. 9B, in synchronization with an image clock which is afixed frequency, light is emitted at the print level P(Idrv+Ib) everypixel (every dot) in the non-image portion at a predetermined ratio(minute pulse width corresponding to weak emission intensity). In FIG.9B, the light amount of the weak emission level (shaded portions) isrealized. In this embodiment, light is always constantly emitted at theweak emission level P(Ib), and the resultant emission intensity is setas the emission intensity of the weak emission level.

The automatic light intensity adjustment for laser is performed in anon-image region (outside an effective region of the photosensitivedrum), such as between scanning lines. However, as the image formingapparatus or the light scanning device becomes more compact, a ratio ofan image region by one scanning in the light scanning device increases,with the result that a ratio of a time period for the non-image regiondecreases. Even in such a case, according to the timing chart of FIG. 8,the automatic light intensity adjustment, which is executed when the SH2signal is enabled, is executed after the output of the horizontalsynchronization signal /BD. Accordingly, the automatic light intensityadjustment can be continued even at a timing at which laser scanningreaches a margin portion of a sheet.

Referring back to the description of FIG. 8, the engine controller 122inputs an enable signal instruction to the enable terminal of the buffer125 by the Venb signal at t3 after a predetermined period of time haselapsed since the output timing (t0 or t1) of the horizontalsynchronization signal /BD. With this, an image mask is released. Inresponse to the enable signal instruction to the enable terminal, theVIDEO signal is output from the video controller 123 from t3 after apredetermined period of time has elapsed since the output timing (t0 ort1) of the horizontal synchronization signal /BD. The LD 107 performslaser emission at the emission level P(Ib+Idrv) for printing, and laserscanning is performed by the optical scanning device, which has beendescribed with reference to FIG. 4. Note that, the weak emission region(t1 to t6), in which light is emitted at the emission intensity of theweak emission level, is larger than the largest image region between t3and t4 in which scanning is performed by the VIDEO signal, and henceweak emission is performed in a region larger than a region between thepaper end portion timings. The weak emission is performed also in thenon-image portion in the region of the VIDEO signal.

FIG. 9C illustrates how the LD 107 emits light when the VIDEO signal isoutput from the video controller 123. In the PWM type weak emission, tothe emission intensity (emission period) of the weak emission level inone pixel described with reference to FIG. 9B, emission at the sameprint level P(Idrv+Ib) is added. In this embodiment, as illustrated inFIG. 9C, PWM emission obtained by pulse width modulation is superimposedon the weak emission level P(Ib) at which light is always emitted (FIG.9A). From FIG. 9C, in the weak emission, radiation noise caused by theweak emission operation can be suppressed to be low as compared to thecase where PWM type weak emission is performed as illustrated in FIG.9B.

Referring back to the description of the timing chart of FIG. 8, thevideo controller 123 scans, in accordance with the VIDEO signal, dots ofa laser beam with respect to the image region of the photosensitive drumuntil t4 after a predetermined period of time has elapsed since theoutput timing (t0 or t1) of the horizontal synchronization signal /BD.The section from t3 to t4 corresponds to a toner image formation region(electrostatic latent image formation region) and corresponds to anemission section in which laser emission is performed by the LD 107. Atthe same timing, the engine controller 122 inputs a disable signalinstruction to the enable terminal of the buffer 125 by the Venb signalfrom t4 after a predetermined period of time has elapsed since theoutput timing (t0 or t1) of the horizontal synchronization signal /BD.This finishes the image mask release period. In other words, the otherperiods correspond to the image mask periods.

At t6 after a predetermined period of time has elapsed since the outputtiming (t0 or t1) of the horizontal synchronization signal /BD, theengine controller 122 turns OFF the switching circuit 116 by the Basesignal, to thereby finish the weak emission.

At this time, the paper end portion timing is t5, and there is arelationship of t4<t5<t6. The paper end portion timing means a timing atwhich laser irradiation from the LD 107 is performed at a position ofthe belt (intermediate transfer belt), which is coincident with the edgeof recording paper on the side parallel to the conveyance direction ofthe recording paper. In FIG. 8, the end timing t6 of the weak emissionis finished before a polygon end portion timing tp (timing of shift fromone surface of the polygon mirror 133 to another surface). However, theweak emission may be set so as to last until t7 (as indicated by brokenline of FIG. 8).

In this manner, the automatic light intensity adjustment for the weakemission level can be performed in a region (between t1 and t6) which iswider than the image region (between t3 and t4) and wider than theregion between the paper end portions (between t2 and t5).

From t7 after a predetermined period of time has elapsed since theoutput timing (t0 or t1) of the horizontal synchronization signal /BD,the engine controller 122 repeatedly executes the processing describedabove for the timing after t2. With this, when a print job is executedin response to a print request from the outside, various kinds of APCcan be performed multiple times efficiently. Regarding the executionfrequency of the APC, the APC may be performed every laser scanning,every page (only scanning of the first line per page), or everypredetermined number of (two or more) laser scannings.

According to the timing chart of FIG. 8, the following effects can beobtained. The emission at the weak emission level (non-image portionweak emission level) is performed at such a level that a developingmaterial such as toner is not adhered by charging on the photosensitivedrum by laser irradiation. Accordingly, the setting of the emissionintensity of the weak emission level (non-image portion weak emissionlevel) can be performed at the timing of the non-image region includingthe effective image region of the photosensitive drum (before the imageregion). With this, a longer time period of the two-level APC can besecured. The processing of the timing chart of FIG. 8 is executedmultiple times in each job, and hence the light amount for weak emissioncan be adjusted multiple times in each job. Throughout one job, thecharge potential Vd can be appropriately maintained, and, as a result,the reversal fog or the positive fog can be suppressed. In the timingchart of FIG. 8, the description has been made of P(Ib) and P(Idrv+Ib).However, if P(Ib) and P(Idrv+Ib) are replaced with P(Ib+Ibias) andP(Idrv+Ib+Ibias), respectively, the same effects can be achieved also bythe circuit of FIG. 7.

In the description of FIG. 8, the APC for P(Idrv) and the APC for P(Ib)have been described. However, if the APC for P(Ib) is performed earlier,the APC for P(Ib+Idrv) can be performed as well. Specifically, the APCfor P(Ib) is first executed. After that, the engine controller 122 setsthe sample/hold circuit 112 to a hold period by the SH2 signal, andturns ON the switching circuit 116 by the input signal Base. That is,the LD 107 is caused to perform bias emission (in laser emissionregion). Then, at the same time, the engine controller 122 sets thesample/hold circuit 102 to a sampling state, and turns ON the switchingcircuit 106 by the Data signal similarly to the above-mentionedembodiment, to thereby cause the LD 107 to perform full emission. Undera state in which the LD 107 is in the full emission state, the emissionintensity of the LD 107 is monitored by the PD 108. A monitor currentIm1′, which is proportional to the actual emission intensity, isgenerated, and the monitor current Im1′ is caused to flow through thecurrent voltage conversion circuit 109 to be converted into a monitorvoltage Vm1′. The current amplifier circuit 104 controls a drive currentIdrv′ based on a current Io1′ flowing through the reference currentsource 105 so that the monitor voltage Vm1′ matches a first referencevoltage Vref11′, which is a target value. In this case, the referencevoltage Vref11′ is a voltage value corresponding to P(Ib+Idrv). Idrv′ isa difference between a current for emitting light at the light amount ofP(Ib+Idrv) and a current for emitting light at the light amount ofP(Ib).

Regarding the execution timing, the APC for P(Ib+Idrv) may be executedat, for example, a timing of the APC for P(Idrv) described withreference to FIG. 8. The timing of the APC for P(Ib) needs to precedethe APC for P(Ib+Idrv). However, it is conceivable to perform the APCfor P(Ib) before forced emission upon detection of the horizontalsynchronization signal /BD. Although the description has been made ofP(Ib) and P(Idrv+Ib), if P(Ib) and P(Idrv+Ib) are replaced withP(Ib+Ibias) and P(Idrv+Ib+Ibias), respectively, the same effects can beachieved also by the circuit of FIG. 7.

In the description of FIG. 8, the APC for P(Idrv) and the APC for P(Ib)are respectively executed, but the present invention is not limited tothis form. For example, the APC for P(Ib+Idrv) may be performed insteadof the APC for P(Ib). Specifically, after the execution of the APC forP(Idrv), the sample/hold circuit 102 is set to a hold period(non-sampling period) by the SH1 signal instructed by the enginecontroller 122, and the switching circuit 106 is turned ON. At the sametime, the sample/hold circuit 112 is set to an APC operation period bythe SH2 signal, and the switching circuit 116 is turned ON by the inputsignal Base. Under a state in which the LD 107 is in the full emissionstate, the emission intensity of the LD 107 is monitored by the PD 108.A monitor current Im2′ (Im1<Im2′), which is proportional to the actualemission intensity, is generated, and is caused to flow into the currentvoltage conversion circuit 109 and converted into a monitor voltageVm2′. The current amplifier circuit 114 controls the drive current Ibbased on a current Io2′ flowing through the reference current source 115so that the monitor voltage Vm2′ matches a potential Vref21′, which is atarget value and is the sum of the first reference voltage and thesecond reference voltage. When the SH2 signal is turned OFF and thesample/hold circuit 112 is set to a hold state, a voltage correspondingto the drive current Ib is charged into the capacitor 113. During anon-APC operation, that is, during a period in which the sample/holdcircuit 112 is in the hold period (non-sampling period) and the Basesignal is ON, the LD 107 enters the full emission state whose lightamount corresponds to the drive current Ib.

For example, the following modified example is possible. For example,first, an automatic light intensity adjusting circuit formed by the samecomponents as the comparator circuit 101, the sample/hold circuit 102,the hold capacitor 103, the current amplifier circuit 104, the referencecurrent source 105, and the switching circuit 106 is added. Theautomatic light intensity adjusting circuit is added so that an outputof the switching circuit is connected directly beneath to the LD 107 andthat a negative terminal of a comparator circuit corresponding to thecomparator circuit 101 is connected to the current voltage conversioncircuit 109. To the negative terminal of the comparator circuitcorresponding to the comparator circuit 101, a voltage valuecorresponding to the drive current Idrv+Ib described in the embodimentis set in advance as a reference voltage Vref01. In this case, theengine controller 122 turns ON the switching circuit and turns OFF theinput signal Base and the Ldry signal. The sampling described here maybe applied to, for example, a period between t2 and t1 of FIG. 8. Theobtained output of the sample/hold circuit (hold capacitor output) isinput to the engine controller 122 via an A/D port (not shown), and istemporarily stored in the RAM as V_(Idrv+Ib). Subsequently, the enginecontroller 122 turns OFF the switching circuit of the added automaticlight intensity adjusting circuit and the switching circuit 116, tothereby execute APC for P(Idrv). A detailed operation thereof is asdescribed above. The obtained output of the sample/hold circuit 102(hold capacitor output) is input to the A/D port (not shown), and istemporarily stored in the RAM as V_(Idrv). The CPU of the enginecontroller 122 calculates V_(Ib) from a difference between V_(Idrv+Ib)and V_(Idrv), which are stored in the RAM, and inputs (sets) thecalculated voltage value to the positive terminal of the currentamplifier circuit 114 via a D/A port (not shown). The sampling describedhere may be applied to, for example, a period between t1 and t2 of FIG.8. In this case, the comparator circuit 111, the sample/hold circuit112, and the like are practically unnecessary.

According to the modified example, the automatic light intensityadjustment for P(Ib) can be executed by an indirect method instead of adirect method. The description above relates to P(Ib) and P(Idrv+Ib).However, if P(Ib) and P(Idrv+Ib) are replaced with P(Ib+Ibias) andP(Idrv+Ib+Ibias), respectively, the same effects can be achieved also bythe circuit of FIG. 7.

The system for performing exposure (emission) by the laser diode 107 hasbeen described as an example, but the present invention is not limitedthereto. For example, the present invention can be carried out also in asystem including an LED array as an exposure unit. Specifically, a VIDEOsignal may be input to a driver for driving each LED light emittingelement, and the processing of the flow chart described above may beexecuted.

In the above, the configuration of the image forming apparatus has beendescribed. Hereinafter, based on the configuration illustrated in FIGS.1A to 9C, referring to FIGS. 11 to 13, description is made ofperforming, by each exposure device (light irradiation unit), weakemission to a position at which the toner image is not visualized.Further, description is made of performing, by each exposure device,general emission to a position at which the toner image is visualized.In the general emission, in addition to the light amount for the weakemission, a light amount based on image data for image formation isfurther applied. Here, description is made of an embodiment in whichrespective target levels of the emission intensities P(Ib) andP(Idrv+Ib) for the weak emission and the general exposure are changed inassociation with the lifetime of the photosensitive drum. In thefollowing description, as a representative example, the configurationand operation of the exposure device 31 a of the first image formingstation a are mainly described. Note that, the same configuration andoperation are applied also in the exposure devices 31 b to 31 d of thesecond to fourth image forming stations, respectively.

(Regarding Need for Correction of Weak Emission Intensity)

Referring to FIG. 10A, problems related to a difference inphotosensitive drum film thickness are described. When thephotosensitive drum 1 is continuously used, the surface of thephotosensitive drum is deteriorated due to the discharge of the chargingroller 2. Further, the surface of the photosensitive drum is worn bybeing brought into sliding contact with the cleaning device 5, therebyreducing the film thickness thereof. When photosensitive drums withvarying usage (for example, cumulative number of rotations) are mixed,the film thicknesses of the respective photosensitive drums fluctuate.Under this state, with the shared high voltage power supply asexemplified in FIGS. 3A and 3B, the constant charge voltage Vcdc isapplied to the multiple photosensitive drums. In this case, generally,the potential difference generated at an air gap between the chargingroller 2 and the photosensitive drum 1 varies, and the charge potentialVd on the surface of the photosensitive drum fluctuates. Specifically,the photosensitive drum which has performed image formation lessfrequently has a larger film thickness, and thus the absolute value ofthe charge potential Vd on the surface of the photosensitive drumdecreases. The photosensitive drum with a larger cumulative number ofrotations has a smaller film thickness, and thus the absolute value ofthe charge potential Vd on the surface of the photosensitive drumincreases.

For example, when the development potential Vdc and the charge potentialVd are set so that a back contrast Vback (=Vd−Vdc), which is a contrastbetween the development potential Vdc and the charge potential Vd, is ina desired state in the photosensitive drum having a larger thickness, asillustrated in FIG. 10A, the following problems arise. That is, in theimage forming station including the photosensitive drum having a smallerfilm thickness, the absolute value of the charge potential Vd increases(Vd Up), and the back contrast Vback increases. When the back contrastVback increases, toner which has been unable to be charged to theregular polarity (in the case of reversal development as thisembodiment, toner charged not to the negative polarity but to 0 orpositive polarity) is transferred onto a non-image portion from thedeveloping roller, to thereby cause fog.

Further, in the image forming station including the photosensitive drumhaving a smaller film thickness, the charge potential Vd increases, andhence in a configuration in which the exposure intensity is constant,the exposure potential Vl (VL) also increases (Vl Up). Therefore, adevelopment contrast Vcont (=Vdc-Vl), which is a difference valuebetween the development potential Vdc and the exposure potential Vl(VL), decreases, and hence the toner cannot be sufficientlyelectrostatically-transferred onto the photosensitive drum from thedeveloping roller, which causes easy occurrence of density reduction ina solid black image.

Meanwhile, as illustrated in FIG. 10B, when the development potentialVdc and the charge voltage Vcdc are fixed and the exposure intensity ischanged from E1 to E2(>E1), owing to the individual control of eachexposure intensity, the development contrast Vcont, which is thedifference value between the development potential Vdc and the exposurepotential Vl (VL), is controllable to be substantially constant.Therefore, the density can be maintained constant. However, the backcontrast Vback, which is the contrast between the development potentialVdc and the charge potential Vd, increases, and hence the problem of thefog occurrence still remains.

(Regarding Correction of Emission Intensity of Weak Emission)

In this embodiment, for example, even in the case of the power supplyconfiguration exemplified in FIGS. 3A and 3B, occurrence of the fog anddensity reduction can be suppressed with a simple configuration. Withreference to a flowchart illustrated in FIG. 11, processing ofperforming the following correction is described. The correction isperformed by changing, in association with the remaining lifetime of thephotosensitive drums 1 a to 1 d, a weak exposure amount E₀ of each ofthe laser diodes 107 a to 107 d in a background portion (non-imageportion) to which toner is not to be adhered. That is, the targetvoltage Vref21 of the emission level for weak emission is changed inassociation with the remaining lifetime of each of the photosensitivedrums 1 a to 1 d. The scanning speed of the optical scanning device ofFIG. 4 is constant.

In S101, the engine controller 122 reads information on the cumulativenumber of rotations of the photosensitive drum 1 as information onremaining lifetime of the photosensitive drum 1 from the storage memberof each of the image forming stations. The storage member of each of theimage forming stations refers to the memory tag (not shown) provided toeach of the image forming stations a to d. A storage section whichstores information on remaining lifetime of each of the photosensitivedrums 1 is not limited to the storage member of each of the imageforming stations. For example, the information read from the storagemember of each of the image forming stations may be temporarily storedin another storage section, and since then, the information stored inthe another storage section may be read and updated. In this case, atthe time of turning OFF the power of the apparatus main body orfinishing the print job, the information of another storage section isreflected to the storage section of each of the image forming stations.

The information on remaining lifetime of the photosensitive drum 1 canbe restated as information related to the usage of the photosensitivedrum 1 in terms of the amount of rotation and the amount of use. Asdescribed with reference to FIG. 2, the information on remaininglifetime of the photosensitive drum 1 can be also restated asinformation related to photosensitivity characteristics (EV curvecharacteristics) of the photosensitive drum 1. All expressions have thesame meaning. As a modified example of the information on remaininglifetime of the photosensitive drum, other than the information on thecumulative number of rotations of the photosensitive drum, differentinformation relative to the film thickness of the charge transportinglayer 24 a of the photosensitive drum can be given. Examples of thedifferent information include information on the number of rotations ofthe intermediate transfer belt, information on the number of rotationsof the charging roller, and information on the number of printed sheets,which takes account of the sheet size. A unit for directly detecting thefilm thickness of the photosensitive drum 1 may be provided so as tocorrespond to each of the photosensitive drums 1, and detection resultsthereof may be used as the information on remaining lifetime of each ofthe photosensitive drums 1. A value of a charge current flowing throughthe charging roller 2, a motor drive time of a motor for driving thephotosensitive drum 1, and a drive time of a motor for driving thecharging roller 2 may be used for the information on remaining lifetimeof the photosensitive drum 1.

In S102, the engine controller 122 refers to a table shown in FIG. 12,which determines the correspondence relationship between the cumulativenumber of rotations of the photosensitive drum 1 (photosensitive drumusage) and parameters for the general exposure. The information acquiredin S101 may vary among the respective photosensitive drums. Therefore,the engine controller 122 refers to the table shown in FIG. 12 for eachof the photosensitive drums. The engine controller 122 sets the exposureparameter of the general exposure amount for each of the laser diodes107 a to 107 d based on the information on the cumulative number ofrotations acquired in S101. The exposure parameter corresponds to Vref11of FIGS. 5 and 7. With the processing in S102, the engine controller 122acquires a laser emission setting for setting the exposure potential Vl(VL) of each of the photosensitive drums 1 to a target potential or apotential within an allowable range regardless of the sensitivitycharacteristics (EV curve characteristics) of each of the photosensitivedrums 1. When the laser diodes 107 a to 107 d perform general emissionwith this acquired setting, variability of post-exposure potentials Vl(VL) of the respective multiple photosensitive drums 1 after generalexposure are at least reduced. The target exposure potentials of therespective photosensitive drums 1 are basically the same orsubstantially the same, but depending on cases, the target exposurepotentials may be individually set in accordance with thecharacteristics of the respective photosensitive drums 1. When the term“exposure” is used for the parameter, the term is used from thestandpoint that the exposure is performed with respect to thephotosensitive drum. When the exposure is performed with respect to thephotosensitive drum, there exists an emission side correspondingthereto. Therefore, when the term “exposure” is used for the parameter,it can be said that the parameter is also a parameter for “emission.”

The operation performed by the engine controller 122 in S102 isdescribed in more detail. The engine controller 122 first sets, by a PWMsignal instruction, values of emission brightness (mW), which correspondto the acquired accumulated information of the respective photosensitivedrums 1, to Vref11 a to Vref11 d, respectively. FIG. 12 shows theemission brightness value (mW) for the purpose of description, butactually, the engine controller 122 sets, by the PWM signal instruction,voltage values/signals, which correspond to the emission brightnessvalues, as Vref11 a to Vref11 d, respectively. Further, the enginecontroller 122 sets the % (PWM) value of the general exposure (density0%) in FIG. 12 to PW_(MIN), and sets the PWM value of the generalexposure (100%) to PW₂₅₅. The engine controller 122 sets a pulse widthwith respect to image data with an arbitrary gradation value n (=0 to255) based on Expression (1) below.PW_(n) =n×(PW₂₅₅−PW_(MIN))/255+PW_(MIN)  Expression (1)

According to Expression (1), PW₀=PW_(MIN) is obtained when n=0, andPW₂₅₅ is obtained when n=255. In the following steps, when emission forimage data with the arbitrary gradation value n is instructed from theoutside, the engine controller 122 instructs a voltage value/signalcorresponding to the set pulse width (PW_(n)) as a VIDEO signal a. Thesame applies to VIDEO signals b to d. Expression (1) assumes a case ofan 8-bit multivalued signal, but in a case of an arbitrary m-bitmultivalued signal, such as a 4-bit, 2-bit, or 1-bit (binary)multivalued signal, the following may be applied. That is, the pulsewidth at the time of PW_(MIN) may be allocated when the image data is 0,and the pulse width at the time of PW₂₅₅ may be allocated to thegradation value (2^(m)−1).

The subsequent step is described. In S103, the engine controller 122sets, based on the cumulative number of rotations of the photosensitivedrum 1, Vref21 as a parameter (emission brightness (mW) in FIG. 12) forthe laser emission intensity E₀ in weak exposure. Also in S103, theengine controller 122 refers to the table shown in FIG. 12 for each ofthe photosensitive drums. Specifically, the engine controller 122 readsthe Vref21 values (PWM values) corresponding to the cumulativeinformation acquired in S101 for the respective photosensitive drums,and sets the Vref21 values to Vref21 a to Vref21 d, respectively. Withthis processing in S103, the engine controller 122 acquires a settingfor setting the charge potential Vd of each of the photosensitive drums1 to a target potential (value of post-correction charge potentialVd_bg) or a potential within an allowable range regardless of thesensitivity characteristics (EV curve characteristics) of thephotosensitive drum. The LD driver 130 performs the APC with thisacquired setting, and under this control, the laser diodes 107 a to 107d are caused to perform weak emission. Thereby, variability ofpost-correction charge potentials at the background portions (non-imageportion) of the respective multiple photosensitive drums 1 are at leastreduced. The target exposure potentials (corresponding to Vref11 values)of the respective photosensitive drums are basically the same orsubstantially the same, but depending on cases, the target exposurepotentials may be individually set in accordance with thecharacteristics of the respective photosensitive drums 1.

With the processing of S102 and S103, it is possible to set the exposureamount of the weak exposure (weak emission) and the exposure amount ofthe general exposure (general emission) appropriately in associationwith the remaining lifetime of each of the photosensitive drums. In S102and S103, a form in which the engine controller 122 refers to the tableof FIG. 12 is described, but the present invention is not necessarilylimited to this form. For example, the CPU in the engine controller 122may compute a calculation expression. Alternatively, the CPU may performthe computation to obtain desired setting values (Vref11 a to Vref11 dor Vref21 a to Vref21 d) from the parameter of the remaining lifetime ofthe photosensitive drum 1 (for example, the cumulative number ofrotations of the photosensitive drum). All values computed by Expression(1) may be stored in advance in a table, and the engine controller 122may refer to the table at each time. The memory tag (not shown) maystore multiple types of EV curves corresponding to the respective statesof usage of the photosensitive drum 1 as illustrated in FIG. 2. In thiscase, the engine controller 122 specifies the EV curve in accordancewith the acquired information on the usage of the photosensitive drum 1.Further, the engine controller 122 computes the necessary exposureamount (μJ/cm²) from the specified EV curve and a desired photosensitivedrum potential. Further, the engine controller 122 computes, from theexposure amount (μJ/cm²) determined at each time, the emissionbrightness, the pulse width at the time of weak exposure, and the pulsewidth at the time of general exposure. The engine controller 122 setsthe results thereof as the parameters corresponding to S102 and S103.

Referring back to the description of FIG. 11, in S104, in response tothe control instruction from the engine controller 122, the respectivemembers execute the series of image formation operation and controldescribed with reference to FIG. 1A. In S105, the engine controller 122measures the number of rotations of each of the photosensitive drums 1 ato 1 d, which has been rotated in the series of image formation. Thismeasurement processing is performed in order to update the usage of thephotosensitive drum 1. The processing of S105 is actually executed inparallel with the processing of S104.

The engine controller 122 determines in S106 whether or not the imageformation has been finished. When it is determined that the imageformation has been finished in S106, the processing proceeds to S107. InS107, the engine controller 122 adds the measurement result of each ofthe photosensitive drums 1 obtained in S105 to the correspondingcumulative number of rotations. In S108, the engine controller 122stores the updated cumulative number of rotations in the non-volatilememory tag (not shown) of each of the image forming stations. In theprocessing of S108, the information on remaining lifetime of thephotosensitive drum 1 is updated. The storage destination in this casemay be another storage section different from the memory tag (not shown)described in S101.

(Description of Table for Correction)

FIG. 12 shows an example of details of a table to which the enginecontroller 122 refers in S102 and S103 of FIG. 11. In the table of FIG.12, the information on remaining lifetime of the photosensitive drum 1(in FIG. 12, “number of rotations of drum” corresponding to thecumulative number of rotations) is associated with the emission controlsetting at the time of weak exposure or at the time of general exposure.This table is stored in the storage section to which the enginecontroller 122 can refer, and for example, stored in the memory tag (notshown) provided in each of the first to fourth (a to d) image formingstations.

In FIG. 12, the exposure amount (μJ/cm²) of the weak exposure and theexposure amount (μJ/cm²) of the general exposure are set in advancebased on the photosensitivity characteristics (EV curve) of the targetphotosensitive drum 1, as exemplified in FIG. 2. Further, in FIG. 12,the setting corresponding to the emission brightness (mW) of the weakexposure is represented by Vref21 and the PWM value correspondingthereto. The setting corresponding to the added emission brightness (mW)for causing the laser diode 107 to emit light at the emission brightness(mW) of the general exposure is represented by Vref11 and the PWM valuecorresponding thereto. The setting of Vref11 is a setting for realizingthe added emission brightness (mW) in FIGS. 5 and 7, and corresponds tothe added emission brightness in FIG. 12. The engine controller 122refers to the table shown in FIG. 12, and thus variability of thesurface potentials in the background portions of the respective multiplephotosensitive drums after charging can be eliminated or at leastreduced. Variability of the post-exposure potentials Vl (VL) of therespective multiple photosensitive drums 1 after the general exposurecan also be eliminated or at least reduced.

In the table exemplified in FIG. 12, both of the emission brightness(mW) during weak exposure and the emission brightness (mW) duringgeneral exposure vary. Through reference to the table of FIG. 12, theengine controller 122 can perform appropriate setting for the generalexposure as well as the weak exposure in association with the cumulativenumber of rotations of the photosensitive drum 1.

In FIG. 12, the emission control parameters at the time of weak exposureand the emission control parameters at the time of general exposure areshown with respect to a certain range of the cumulative number ofrotations of the photosensitive drum 1, but the condition may be set inmore detail. For example, with the CPU of the engine controller 122,from the relationship between the number of rotations of the drum andeach emission control setting value in the table, an appropriateemission control setting value with respect to the arbitrary number ofrotations of the drum may be presumed and calculated (predicted andcomputed). Further, the same applies also in the case of the generalexposure. In this manner, the accuracy of intensity of emission by thelaser diode 107 during weak exposure or that during general exposure canbe further enhanced. Further, with reference to the table of FIG. 12, acase is described, in which both of the weak exposure amount and thegeneral exposure amount linearly increase in accordance with thecumulative number of rotations of the photosensitive drum 1. However,the present invention is not limited thereto. In view of thecharacteristics of the photosensitive drum 1, there may be provided atable in which the exposure amounts non-linearly increase in accordancewith the cumulative number of rotations of the photosensitive drum 1.

(Description of Action and Effect)

With reference to FIG. 10C, an action and effect by processing of theflowchart of FIG. 11 is described. In this embodiment, the filmthickness of the charge transporting layer 24 of the photosensitive drum1 is 20 μm when largest (the photosensitive drum 1 in the initialstate), and the charge potential Vd is about −600 V after passagethrough the charging roller 2 (see FIG. 2). When the cumulative numberof rotations of the photosensitive drum 1 increases and the filmthickness of the charge transporting layer 24 reduces, the chargepotential Vd becomes about −700 V and the charge potential Vd varies byabout −100 V. When the photosensitive drums with varying usage aremixed, or when the photosensitive drums with varying characteristics aremixed, difference is generated in the EV characteristics among thephotosensitive drums.

When the charge transporting layer 24 is thinned, the charge potentialVd increases (see FIGS. 10A and 10B), and hence in the case where theexposure amount of the image portion exposure is set constant, thepost-exposure potential Vl (VL) increases (FIG. 10A). The exposureamount during full emission is increased from E1 to E2 in accordancewith the cumulative number of rotations of the photosensitive drum,which is inversely proportional to the film thickness of the chargetransporting layer 24, and the post-exposure potential Vl (VL) ismaintained substantially constant as the solid line portion in FIG. 10C.Therefore, the development contrast Vcont (=Vdc−Vl), which is adifference value between the development potential Vdc and the exposurepotential Vl (VL), can be maintained at a constant value regardless ofthe film thickness of the charge transporting layer 24 of thephotosensitive drum 1.

Thus, occurrence of image density reduction is suppressed.

As the value of the cumulative number of rotations of the photosensitivedrum 1 increases, the laser light amount during non-image portionexposure is increased from Ebg1 to Ebg2. This is as described withreference to the table of FIG. 12. Even when a constant DC voltage isapplied to the charging rollers 2 a to 2 d, it is possible to correctthe amount of increase (difference between A and B of FIG. 10C) of thecharge potential Vd caused by the film thickness change of the chargetransporting layer 24 of the photosensitive drum 1. With this, asindicated by the bold solid line in FIG. 10C, the post-correction chargepotential Vd_bg in the non-image portion is substantially constantregardless of the film thickness of the charge transporting layer 24.Even when the development potential Vdc is a constant value, the backcontrast Vback, which is a potential difference between the developmentpotential Vdc and the post-correction charge potential Vd_bg, ismaintained constant. This point differs from the case exhibiting onlyE1<E2 as illustrated in FIG. 10B. In this manner, it is possible tosuppress the fog caused when toner which has been unable to be chargedto the regular polarity (in the case of the reversal development, tonercharged not to the negative polarity but to 0 or positive polarity) istransferred onto the non-image portion.

FIG. 13 illustrates image quality evaluation transitions of comparativeexamples and a case where the weak exposure condition is changed by theabove-mentioned system. In FIG. 13, Comparative Example 1 represents acase where no correction is performed for the background portionpotential Vd by the weak exposure or the like. Comparative Example 2represents a case where the high voltage power supply circuit controlsthe charge potential Vcdc to perform correction of the backgroundportion potential Vd. The term “correction” refers to, for example, acorrection in which Vd_bg in FIG. 10C is set as a target potential.

FIG. 13 illustrates the transition of the fog amount M. In ComparativeExample 1 of FIG. 13, the charge potential Vd increases along with theincrease of the cumulative number of rotations of the photosensitivedrum, and hence the reversal fog, which is caused by the increase of thepotential difference between the background portion potential and thedevelopment potential, is significantly worsened.

In Comparative Example 2 of FIG. 13, the reversal fog is not worsened.However, along with the progress of usage, the charging roller 2 iscontaminated and the fog occurs locally at a portion at which thebackground portion potential is low, and thus the total fog amount tendsto increase. The reason is as follows. In the control of the chargepotential Vcdc, in a case of this embodiment, the charge potential Vcdcis set so as to set the surface of the photosensitive drum to, forexample, −700 V, but in Comparative Example 2, the charge potential Vcdcis set so as to set the surface of the photosensitive drum to, forexample, −600 V. When the charge potential Vcdc is small, the chargepotential contrast between the contaminated portion and theuncontaminated portion of the charging roller 2 tends to increase. Thatis, the tendency of charge ability reduction in the contaminated portionof the charging roller 2 more markedly appears in the case of a smallcharge potential Vcdc than the case of a large charge potential Vcdc.

According to this embodiment, the charge potential (background portionpotential) is maintained constant to suppress worsening of the reversalfog, and in addition, the exposure amount E₀ for weak exposure isincreased to ensure a sufficient smoothing effect. In addition to thiseffect, without causing reduction in uniformity of the charge potentialdue to the contamination of the charging roller and the like, thebackground portion potential can be formed. Therefore, it is possible totake an effective countermeasure against increase in background portionpotential and reduction in uniformity caused along with the progress ofthe usage degree. The background portion potential is maintainedconstant in each of the image forming stations, and hence there is anadvantage that, even when a voltage is supplied from the same powersupply to the respective developing rollers, the worsening of the fogcan be suppressed.

(Modified Example)

In the description above, in both cases of FIGS. 3A and 3B, a one sharedpower supply (corresponding to transformer 53) is used as a high voltagepower supply for the charging rollers 2 and the developing rollers 43.However, as is clear from the description of FIGS. 10A and 10B, thepresent invention is also effective in a case power supply control forcharging cannot be performed independently among colors and power supplycontrol for development cannot be performed independently among colors.That is, one power supply (corresponding to one transformer) for themultiple charging rollers, and one power supply (corresponding to onetransformer) for the multiple developing rollers may be provided. Withthe description of “a first power supply” and “a second power supply,”the respective power supplies are distinguished. In this case, a voltageoutput from the one power supply for charging (first power supplyvoltage) or a voltage obtained by converting the above-mentioned voltageby a converter (first conversion voltage) is input to the respectivecharging rollers 2 a to 2 d. A voltage output from the one power supplyfor development (second power supply voltage) or a voltage obtained byconverting the above-mentioned voltage by a converter (second conversionvoltage) are input to the respective developing rollers 43 a to 43 d. Asdescribed with reference to FIGS. 3A and 3B, a variety of voltages maybe input to the individual rollers (charging rollers and developingrollers). For example, the power supply voltage (first power supplyvoltage or second power supply voltage) of each of the power supplies (afirst power supply and a second power supply) may be directly input tothe charging rollers 2 a to 2 d or the developing rollers 43 a to 43 d.Alternatively, a voltage obtained by converting the power supply voltageof each of the power supplies by a converter (first conversion voltageor second conversion voltage) may be at least one of divided and droppedby an electronic element having fixed voltage drop characteristics, andthe obtained voltage (first voltage or second voltage) may be input tothe charging rollers 2 a to 2 d or the developing rollers 43 a to 43 d.

In the description above, when the voltage is dropped/boosted, a case ofperforming at least one of dividing and dropping the voltage by theelectronic element having the fixed voltage drop characteristics hasbeen described. However, the processing of weak exposure in theflowchart of FIG. 11 is also effective in a case where a DC-DC converterhaving a specific function is provided to the individual charging rolleror developing roller. That is, in the case of the situation illustratedin FIG. 10A, when the voltage conversion ability of the DC-DC converteris insufficient, Vd_bg illustrated in FIG. 10C cannot be realized onlyby the voltage conversion ability thereof. In such a case, the formationof the potential lacking only by the DC-DC converter is compensated forby the weak exposure processing, and thus the charge potential Vd_bg maybe realized.

According to the embodiments described above, it is possible to solveproblems caused by the charge potential of the photosensitive drum bycoping with variability or variations in photosensitivitycharacteristics (EV curve characteristics) of the photosensitive drumsin the apparatus, and appropriately controlling the charge potential ofeach of the photosensitive drums with a simpler configuration.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2011-114859, filed May 23, 2011, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A color image forming apparatus which has a lightirradiation unit irradiating a charged photosensitive member with lightto form an electrostatic latent image, the color image forming apparatuscomprising: a light emitting element emitting light and provided in thelight irradiation unit; a laser driving unit causing the light emittingelement to emit light in accordance with an input of print data, thelaser driving unit causing the light emitting element to emit light of alight amount at a first emission level for visualizing a toner imageonto a first area where the toner image is to be visualized on thecharged photosensitive member and to emit light of a light amount at asecond emission level for weak emission onto a second area where thetoner is not to be adhered to the charged photosensitive member, whereinthe laser driving unit supplies a drive current for the first emissionlevel to the light emitting element so that the light emitting elementemits light of the light amount at the first emission level, and thelaser driving unit supplies a drive current for the second emissionlevel to the light emitting element so that the light emitting elementemits light of the light amount at the second emission level; anacquiring unit acquiring information of an integrated number ofrotations of the photosensitive member; and a drive current adjustingunit adjusting the drive current for the second emission level, thedrive current adjusting unit changing a magnitude of the drive currentfor the second emission level in accordance with the information of theintegrated number of rotations of the photosensitive member acquired bythe acquiring unit.
 2. The color image forming apparatus according toclaim 1, wherein the drive current adjusting unit changes a magnitude ofthe drive current for the first emission level in accordance with theinformation of the integrated number of rotations of the photosensitivemember acquired by the acquiring unit.
 3. The color image formingapparatus according to claim 2, wherein the drive current adjusting unitis configured to increase the magnitude of the drive current for thefirst emission level as the integrated number of rotations of thephotosensitive member increases.
 4. The color image forming apparatusaccording to claim 1, wherein the magnitude of the drive current for thefirst emission level and the magnitude of the drive current for thesecond emission level are independently controllable by the drivecurrent adjusting unit.
 5. The color image forming apparatus accordingto claim 1, further comprising: a power supply which is a single powersupply, wherein the power supply supplies power to a charging unit whichcharges the photosensitive member, and to a developing unit whichvisualizes a latent image formed on the photosensitive member withtoner, wherein the charging unit is supplied with one of a power supplyvoltage output from the power supply, a conversion voltage obtained byconverting the power supply voltage by a converter, and a voltageobtained by at least one of dividing and dropping one of the powersupply voltage and the conversion voltage by an element having fixedvoltage drop characteristics, and wherein the developing unit issupplied with one of a conversion voltage obtained by converting thepower supply voltage by a converter, and a voltage obtained by at leastone of dividing and dropping one of the power supply voltage and theconversion voltage by an element having fixed voltage dropcharacteristics.
 6. The color image forming apparatus according to claim1, further comprising: a first power supply which is a single powersupply, wherein the first power supply supplies power to a plurality ofcharging units which charge a plurality of photosensitive members; and asecond power supply which is a single power supply, wherein the secondpower supply supplies power to a plurality of developing units whichvisualize latent images formed on the plurality of photosensitivemembers with toner, wherein the plurality of charging units is suppliedwith one of a first power supply voltage output from the first powersupply, a first conversion voltage obtained by converting the firstpower supply voltage by a converter, and a first voltage obtained by oneof dividing and dropping one of the first power supply voltage and thefirst conversion voltage by an element having fixed voltage dropcharacteristics, and wherein the plurality of developing units issupplied with one of a second power supply voltage output from thesecond power supply, a second conversion voltage obtained by convertingthe second power supply voltage by a converter, and a second voltageobtained by one of dividing and dropping one of the second power supplyvoltage and the second conversion voltage by an element having fixedvoltage drop characteristics.
 7. The color image forming apparatusaccording to claim 1, wherein the drive current adjusting unit isconfigured to increase the magnitude of the drive current for the secondemission level as the integrated number of rotations of thephotosensitive member increases.
 8. The color image forming apparatusaccording to claim 1, wherein the first area and the second area areincluded in an area on the photosensitive member where the latent image,which is visualizable as the toner image, can be formed.
 9. An imageforming apparatus which has a light irradiation unit irradiating acharged photosensitive member with light to form an electrostatic latentimage, the image forming apparatus comprising: a light emitting elementemitting light and provided in the light irradiation unit; a laserdriving unit causing the light emitting element to emit light inaccordance with an input of print data, the laser driving unit causingthe light emitting element to emit light of a light amount at a firstemission level for visualizing a toner image onto a first area where thetoner image is to be visualized on the charged photosensitive member andto emit light of a light amount at a second emission level for weakemission onto a second area where the toner is not to be adhered to thecharged photosensitive member, wherein the laser driving unit supplies adrive current for the first emission level to the light emitting elementso that the light emitting element emits light of the light amount atthe first emission level, and the laser driving unit supplies a seconddrive current for the second emission level to the light emittingelement so that the light emitting element emits light of the lightamount at the second emission level; an acquiring unit acquiringinformation of an integrated number of rotations of the photosensitivemember; and a drive current adjusting unit adjusting the drive currentfor the second emission level, the drive current adjusting unit changinga magnitude of the drive current for the second emission level inaccordance with the information of the integrated number of rotations ofthe photosensitive member acquired by the acquiring unit.
 10. The imageforming apparatus according to claim 9, wherein the drive currentadjusting unit changes a magnitude of the drive current for the firstemission level in accordance with the information of the integratednumber of rotations of the photosensitive member acquired by theacquiring unit.
 11. The image forming apparatus according to claim 10,wherein the drive current adjusting unit is configured to increase themagnitude of the drive current for the first emission level as theintegrated number of rotations of the photosensitive member increases.12. The image forming apparatus according to claim 9, wherein themagnitude of the drive current for the first emission level and themagnitude of the drive current for the second emission level areindependently controllable by the drive current adjusting unit.
 13. Theimage forming apparatus according to claim 9, further comprising: apower supply which is a single power supply, wherein the power supplysupplies power to a charging unit which charges the photosensitivemember, and to a developing unit which visualizes a latent image formedon the photosensitive member with toner, wherein the charging unit issupplied with one of a power supply voltage output from the powersupply, a conversion voltage obtained by converting the power supplyvoltage by a converter, and a voltage obtained by at least one ofdividing and dropping one of the power supply voltage and the conversionvoltage by an element having fixed voltage drop characteristics, andwherein the developing unit is supplied with one of a conversion voltageobtained by converting the power supply voltage by a converter, and avoltage obtained by at least one of dividing and dropping one of thepower supply voltage and the conversion voltage by an element havingfixed voltage drop characteristics.
 14. The image forming apparatusaccording to claim 9, wherein the drive current adjusting unit isconfigured to increase the magnitude of the drive current for the secondemission level as the integrated number of rotations of thephotosensitive members increases.
 15. The color image forming apparatusaccording to claim 9, wherein the first area and the second area areincluded in an area on the photosensitive member where the latent image,which is visualizable as the toner image, can be formed.